Pedicle screws usually serve for the dorsal stabilization of the spinal column in the event of fractures, tumors, inflammations, deformities and degeneratively induced instabilities by means of a transpedicular screw connection. In doing so, pedicle screws are placed in the pedicles of neighboring vertebrae, creating an angularly stable connection between the pedicle screws arranged axially one above the other and an axially extending longitudinal member or bar. In such an arrangement, the pedicle screws and the longitudinal member constitute a vertebral stabilization system.
The pedicle screws are screwed through the pedicle into the vertebral body (corpus vertebrae) between the transverse process (processus transversus) and the spinous process (processus spinosus). To this end, a pedicle screw usually comprises a screw head and an elongated screw shaft provided with an external thread. Whereas the screw head is formed like a tulip for the fixation of the longitudinal member or carries a so-called tulip, the screw shaft serves for anchoring the pedicle screw in the vertebra.
Various geometries are known with regard to the design of the screw shaft.
By way of example, screws are used which have a cylindrical thread and in which both the core diameter and the outer or flank diameter remain constant over the entire longitudinal extension of the screw shaft (cf. FIG. 4A). These screws have a very high pull-out strength which is hardly affected even in the event of turning back the screw in order to correct the position of the screw head, for instance. It is a disadvantage of constant external diameters and core diameters that the thread—which is cut into the bone by the external thread in the entry area between the transverse process and the spinous process—may wear out in case of further driving in the screw, possibly reducing the support in the area of the proximal shaft portion. A further disadvantage lies in the fact that a cylindrical thread, in particular a constant core diameter, is not capable of compacting the cancellous bone material, which is present in the vertebra, between the thread flanks.
Further, it is also known that there are screws comprising a fully conical thread in which both the outer thread diameter and the core diameter uniformly increase from the screw tip up to the screw head. The advantage of this screw geometry is to see in the fact that they have a good grip in the bone and even the proximal shaft portion cuts further into the bone; in this way, a good support is also ensured in the proximal area. However, screws comprising a fully conical thread do not allow for their subsequent fine adjustment, as otherwise the contact of the thread flanks of the screw with the bone's thread which has been excessively widened beforehand in radial direction would get lost over the entire length even in the case of only slightly turning back the screw.
This is why screws comprising a partially conical thread are frequently used, in which the core is conical and the thread is formed to be cylindrical (cf. FIG. 7A). The cylindrical thread ensures a high pull-out strength. Due to the increasingly larger core diameter, a radial pressure on the cancellous bone material is produced between the thread flanks and said bone material is compacted, improving the support. What is more, a screw comprising a partially conical thread also has a higher fatigue strength than a screw with a constant core diameter which is due to the increasing core diameter.
In the case of screws comprising a partially conical thread, however, the thread flanks become increasingly broad and dull toward the screw head; this is why said flanks do not have a cutting effect any more in the proximal area of the screw shaft, but are pressed into the thread turn in radial and axial directions and possibly expand or even burst the thread turn in the proximal area. Also in this case, a slight backward turn may result in the screw becoming loose, as the thread flanks lose the contact with the thread turn (which is expanded in axial direction).
As each of these geometries has advantages and disadvantages, combinations of these geometries are also known, such as e.g. a screw comprising a cylindrical external diameter and a core with alternating cylindrical and conical core diameter portions (cf. FIGS. 5A and 6A).
As already mentioned above, the entry point of a pedicle screw is situated between the transverse process and the spinous process, then extends through the pedicle in the axial direction of the relatively narrow pedicle and opens out into the vertebral body. In the area of the vertebral body and in the area between the entry point and the pedicle, there mainly is soft cancellous bone material, whereas dense cortical bone material can be found in the slim area of the pedicle immediately in the area of the entry point. None of the thread shapes known hitherto satisfies this bone structure of a vertebra and the inhomogenous bone strength.